Forgeable high strength austenitic alloy with copper, molybdeum, columbium-tantalum,vanadium, and nitrogen additions



F. T. EBERLE Feb. 25, 1958 FORGEABLE HIGH STRENGTH AUSTENITIC ALLOY WITH COPPER, MOLYBDENUM, COLUMBIUM-TANTALUM VANADIUM, AND NITROGEN ADDITIONS Filed July 30, 1954 mmmmhm mun-Z: mmaO: 000 9 000- .E ckmaoza mm:

6 r h T 5 T 1m ATTORNEY United States This invention relates to forgeable alloy steels having enhanced stress rupture strength, corrosion resistance, and freedom from embrittlement in extended service, at elevated temperatures and stresses, and, more particularly, to a fully austenitic chrome-nickel-iron alloy steel attaining the foregoing properties with a minimum total alloy content.

For a number of years there has been a steady increase in the superheater outlet temperatures and pressures of vapor generators, with a resulting increase in the efi'iciency and economy of turbines driving electric generators. These temperature and pressure increases required alloy steels to be used in the superheaters, such as stainless steels of the columbium and titanium bearing l8-Cr-8Ni AISI Types 347 and 321. With superheater outlet temperatures of 1050 F., the pressures involved are fre quently substantially in excess of 2000 p. s. i.

With pressures of this order, the superheater tubing must have wall thicknesses of up to /5 for such 18-8 alloys to remain within their allowable working stresses. Such wall thicknesses are undesirable, not only from the standpoint of fabrication problems but also from the standpoints of heat transfer and thermal stress gradients across the wall of the tubing. As a consequence, the increase in superheater outlet temperatures recently has been arrested at substantially the 1100 F. level.

Any further substantial increase in superheater outlet temperatures requires steel alloys capable of practical fabrication into tubing having wall thicknesses acceptable from the fabrication, heat transfer, and thermal stress gradient standpoints, and having long-time strength and corrosion resistance at temperatures in excess of 1350 F. and pressures substantially in excess of 2000 p. s. i. In addition, considering the large quantities of such tubing required in modern vapor generator installations, such alloys must have a low total alloy content in order to be economically feasible for use as superheater tubing.

There are known alloys which have long-time strength at high temperatures but which either have too high an alloy content to be economically practical for use as superheater tubing or are substantially non-forgeable, difficult to forge, or characterized by a loss of desirable properties in long-time service at elevated temperatures.

The present invention is, accordingly, directed to a steel alloy capable of economically practical use as tubing operating at temperatures in excess of 1350 F. and pressures in excess of 2000 p. s. i., and having the lowest possible alloy content, being particularly low or lean in strategically important elements. The invention is particularly directed to such an alloy meeting the following requirements:

(1) Stress-rupture strength, at 1350 F., at least twice that of AISI Type 304 alloys, the most economical steel alloys commercially available for use at such elevated temperatures;

(2) Adequate resistance to corrosion by superheated vapor and combustion gases at 1350 F.;

(3) Adequate hot plasticity, for fabrication into tubing;

(4) Favorable mechanical properties;

(5) Weldability; and

Patented Feb. 25, 1958 (6) Freedom from serious embrittlement in long-time service at such high temperatures.

To meet these requirements, the invention alloy has the following base composition:

Balance iron with the usual impurities.

This base composition is a fully austenitic iron-chromenickel steel alloy of relatively low carbon and silicon content. The chromium content is sufiiciently high for adequate oxidation and corrosion resistance at temperatures of the order of about 1500 F., and yet sufliciently low to suppress sigma-phase formation. The nickel content is sufficient to maintain the alloy structure fully austenitic over a wide range of variation in alloying additions. The fully austenitic, or face-centered lattice, structure is important for maximum sustained high-temperature strength, the low carbon content assures hot plasticity and weldability, and the low silicon content is adequate insurance against micro-fissuring in Welding.

The creep-rupture strength of this base composition is raised by suitable alloy addition designed to produce age hardening processes in the base composition by forming complex carbides or intermetallic compounds which are soluble in the base composition at very high temperatures but insoluble or of limited solubility therein at lower temperatures in the general vicinity of the contemplated use temperature; i. e. of the order of 1350 F., or higher.

in accordance with the present invention, the creep rupture stren th of the base composition is very substantially increased by adding thereto Cu from 2.50% to 3.00%, Mo from 1.20% to 1.70%, CbTa from 0.50% to 2.00%, V from 0.40% to 0.70%, and N from 0.10% to 0.25%. The invention alloy may be classed generally as a 15Cr- 15Ni2.5Cu-1.25Mo-2.0CbTa-O.5V0.15N steel alloy.

In the drawing, the single figure is a graphical comparison, at 1350 R, of the creep rupture strength of the invention alloy and an AISI Type 304 l8Cr-8Ni steel alloy.

In the invention alloy, the chromium content selected had to be high enough to insure adequate resistance to oxidation and scaling at a contemplated maximum use temperature of 1350 F. to 1450 F., and low enough to inhibit or minimize the formation of embrittling sigma phase. A chromium content of 15% to 17% is suitable for effecting these results. It is advisable to hold the chromium content on the low side since chromium, as well as most of the other elements available for strengthening the base composition, is a ferrite former promoting the weak, body-centered cubic lattic structure which has to be compensated by suitably increased additions of the relatively expensive, and strategically important, austenite forming nickel.

With a chromium content of 15% to 17%, a nickel content of 15% is sufiicient to neutralize the ferrite forming tendencies of chromium and the precipitate producing and strengthening additions. A chromium content of 15% to 17% with a nickel content of about 15% is advantageous from the standpoint of creep strength, as investigations demonstrate that an increase in nickel content above 8% and 10%, required to achieve a fully austenitic structure in a 20% chrome-iron alloy, does not improve the creep strength in any significant degree.

Similarly, if the nickel content in such an alloy is held at 15 an increase in the chromium content from 15 to 25% does not add materially to the creep strength.

The carbon content of the invention alloy is carefully r r '9 selected to assure a high strengthening the alloy through formation of complex carbides, yet not so high as to affect adversely forgeabilityand leadto seams in tubing formed from the alloy. For this purpose, the carbon content has a maximum of 0.15% and preferably is held between 0.03% and 0.05%. r

' 'Manganes'e has a beneficial efiect'upon hot working enough carbon content for propertiesdue to its action upon oxygen'and sulphur. It

is also desirable'as an ingredient due to its tendency to form. austenite, although its potency, in this respect, is

- inferior torthat of nickel. 1 Hence, thepreferred manganese'contentis1.75%., which is near the upper end of the, range 'Qfmanganese commonly found in 18-3 type alloy Silicon, is a strong ferrite former, and should be'kept at a low value ,where itis'desired to promote austenite formation. On the other hand, silicon participatesin the formation of strengthening compounds, such as silicides, with columbium and tantalum. It is also a powerful deoxidizer; and enhances resistance to oxidation, at high temperatures, by forming a tightly adherent protective scale, being much more efiective than chromium in'this respect. By combining a low-range chromium content with ahigh-range silicon content, satisfactory scaling resistance, with a, minimum tendency to sigma-phase em- 'brittlemenn is assured. For these reasons, a silicon content of substantially 0.75% is preferred.

As stated, the base composition is strengthened. by alloy additions designed to produce age hardening proc-' esses. For this purpose, the invention allo including such additions, is solution heat treated at a high temperature, such as 2200 1 to 2 300 F, followed by an aging treatment, or by use, at a lower temperature,,such as 1350" F. 'With suitable alloy additions, a fine dispersion of precipitated compounds in the lattice structure of the matrix is achieved. This fine dispersion resists or retards 7 plastic deformation under stress at elevated temperatures,

and thus produceshigh load carrying ability at such elevated temperatures.

In accordance with the present invention, it has been found that,rof available strengthening alloy additions,

tantalum, oolumbium-tantalum, [and molybdenum are r'no st potent in improving the rupture strength, at elevated temperatures, of the base composition. It also appears that pure columbium, in this respect, is not as efiective as tantalum and'columbium-tantalum; Copper andnitrogen, when used with tantalum, or columbium-tantalum,

and molybdenum, are very effective in obtaining optimum stress-rupture strength at high temperatures. Vanadium is also an effective strengthening agent, in this respect, r

particularly when added in combination with nitrogen.

It has been found further'that the strengthening effect of individual additives is not additive and does not even correlate with the relative individual potencies when the additives are added in combination to the base composition. There appearsto he an optimum concentration'for eachelement which appears to differ for difierent com- 7 binations of additives. V r

' Within the composition range previously tabulated, a

preferred composition of a forgeable, high-strength-athigh-temperature alloy embodying the invention, and which is lean in alloy content and economically practical for superheater tubing, is as follows:

C1; 17.00% rnaxirnum. Ni 15 .00% maximum. 'C.., 0.12% maxim-unt Mn..., 2.00% maximum.

Si 0.75% maximum.

1"" i ZOO-3.00%.

Mo LOO-2.00%.

CbTa. 0.50-2.00%. V.... 0.20-100%.

N DAG-0.25%..

Balance iron with the usual impurities.

The preferred percentages of the last five additives are Alloys of substantially this preferred composition have been compounded, solutionheat-treated at 2200 F.

2300 F. and aged at temperatures of the order 0f1300f F.1 500 F, Thesealloysr havethenbeen subjected-to: stress rupture testsgat 135.0 for up to 5,000 hours.

In the accompanying drawing, thestress-rupture curve, plottedon a logarithmiescale with rupture strength in .p. s. i. as ordinates and hours under stress as absciss'ae, 'is given with actual values up to the 5,000 hours point, and extrapolated to 100,000 hours, the test temperature being 1350 F. i

Curves A and A represent; the stress-rupture values of alloys embodying theinvention, while curve B rep resents those of an AISI Type'304 18Cr-8Nialloy; It

will be observed that, at. 1000 hours, the stress-rupture strength of the invention alloys is 18,00020,000 p. s. i., over twice the strength of the Type 304 alloy. At 10,000 hours, the stress-rupture strength offtheinven tion alloys is 12,500-14,500 .p. s. i. as compared to about 4700 p. s. i. for the Type304 alloy. At 100,000 hours, the indicated rupture strength .of the invention alloys is.8,5 00 to 10,500 ps. i., over three times the 2700 p. s. i. value for the Type 304 alloy. The values for the AISI Type 304 alloy are taken from ASTMT-ASME I 'Spec. Tech. Publ. No. 124. 7 1 It will be noted that the invention alloys have stress- -rupture strengths, at'l350" F., over twice that of the 7 Type 304 alloy. Consequently, they can be used to 7 form tubing suitable for prolonged service at such temperature and having reduced wall thicknesses acceptable from :the fabrication, thelrmalstress gradient, andjecon- 7 only standpoints, while having aprolonged stressre- 'sistance at least equal to that of tubing formed from;

the Type 304 alloys; 1 The alloys embodying the positions within the-following ranges: I r

. Percent Cr 14.70-17.00 Ni 14.00 1100 7 c p.041 011 Ma 1.5m 1.85 Si {050 0.88 Cu 2.45-2.50 Mof 1.18 1.50 Cb-Ta, 0.50- 2.06 v 7 ..0.s0- 0.53v

N V o 7 V j V 0.1'1 0.21 Balance iron with the usual impurities.-

'Two' specific alloys falling, within thiSIange are the I Balance ironwith the usual impurities. V r

The stress-rupture'strengthjof alloy #1 is. represente by curve A and that of'alloy #2 by curve A.

invention and represented. bycurves A and A of the drawing have percentage'com Percent Cr 15.00-20.00 Ni 12.00-18.00 C 0.02- 0.15 Mn 0.25- 2.50 Si 0.10- 1.00 Cu 2.00- 3.00 Mo 1.00- 2.00 Cb-Ta 0.50- 2.00 V 0.20- 1.00 N 0.10- 0.25

Balance iron with the usual impurities; said alloy having a rupture strength, after 1000 hours under stress at 1350 F., of at least 18,000 p. s. i.; and after 5000 hours under stress at 1350 F., of at least 16,000 p. s. i.

2. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Cr 17.00% maximum. Ni 15.00% maximum. C 0.12% maximum. Mn 2.00% maximum. Si 0.75% maximum. Cu 2.00-3.00%. M l.002.00%. Cb-Ta 0.502.00%. V 0.20-1.00%. N 0.l0-0.25%.

Balance iron with the usual impurities; said alloy having a rupture strength, after 1000 hours under stress at 1350 F., of at least 18,000 p. s. i.; and after 5000 hours under stress at 1350 F., of at least 16,000 p. s. i.

3. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Cr 17.00% maximum. Ni 15.00% maximum. C 0.12% maximum. Mn 2.00% maximum. Si 0.75% maximum. Cu I 2.50% Mo 1.25% Cb-Ta 2.00% V 0.50% N 0.15%

Balance iron with the usual impurities; said alloy having a rupture strength, after 1000 hours under stress at 1350 F., of at least 18,000 p. s. i.; and after 5000 hours under stress at 1350 F., of at least 16,000 p. s. i.

4. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

H Percent Cr 14.70-17.00 Ni 14.00-15.00 C 0.041- 0.11 M11 1.50- 1.85 Si 0.50- 0.88 Cu 2.45- 2.50 Mo 1.18- 1.50 Cb-Ta 0.50- 2.06 V 0.50- 0.53 N 0.11- 0.21

Balance iron with the usual impurities; said alloy having a rupture strength, after 1000 hours under stress at 1350 F., of at least 18,000 p. s. i.; and after 5000 hours under stress at 1350 F., of at least 16,000 p. s. i.

5. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Percent Cr 14.70 Ni 15.00 C 0.041 Mn 1.85 Si 0.88 Cu 2.45 Mo 1.18 Cb-Ta 2.06 V 0.53 N 0.1 1

Balance iron with the usual impurities; said alloy having a rupture strength, after 1000 hours under stress at 1350 F., of at least 18,000 p. s. i.; and after 5000 hours under stress at 1350 F., of at least 16,000 p. s. i.

6. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300' F.; said alloy having the following composition:

Percent Cr 17.00 Ni 14.00 C 0.11 Mn 1.50 Si 0.50 Cu 2.50 Mo 1.50 Cb-Ta 0.50 V 0.50 N 0.21;

Balance iron with the usual impurities; said alloy hav-- ing a rupture strength, after 1000 hours under stress at 1350 F., of at least 18,000 p. s. i.; and after 5000 hours under stress at 1350 F., of at least 16,000 p. s. i.

References Cited in the file of this patent UNITED STATES PATENTS 2,536,034 Clarke Ian. 2, 1951 2,540,509 Clarke Feb. 6, 1951 FOREIGN PATENTS 670,555 Great Britain Apr. 23, 1952 908,191 France Apr. 2, 1946 478,014 Italy Feb. 12, 1953 

1. A FORGEABLE AUSTENITIC STEEL ALLOY HAVING SUPERIOR STRESS RESISTANCE AND CORROSION RESISTANCE PROPERTIES, AND FREEDOM FROM IMPACT EMBRITTLEMENT, IN EXTENDED SERVICE UNDER STRESS AT TEMPERATURES OF THE ORDER OF 1300* F.; SAID ALLOY HAVING THE FORMULA COMPOSITION: 